A known autostereoscopic display device comprises a two-dimensional liquid crystal display panel having a row and column array of display pixels (wherein a “pixel” typically comprises a set of “sub-pixels”, and a “sub-pixel” is the smallest individually addressable, single-colour, picture element) acting as an image forming means to produce a display. An array of elongated lenses extending parallel to one another overlies the display pixel array and acts as a view forming means. These are known as “lenticular lenses”. Outputs from the display pixels are projected through these lenticular lenses, which function to modify the directions of the outputs.
The lenticular lenses are provided as a sheet of lens elements, each of which comprises an elongate semi-cylindrical lens element. The lenticular lenses extend in the column direction of the display panel, with each lenticular lens overlying a respective group of two or more adjacent columns of display sub-pixels.
Each lenticular lens can be associated with two columns of display sub-pixels to enable a user to observe a single stereoscopic image. Instead, each lenticular lens can be associated with a group of three or more adjacent display sub-pixels in the row direction. Corresponding columns of display sub-pixels in each group are arranged appropriately to provide a vertical slice from a respective two dimensional sub-image. As a user's head is moved from left to right a series of successive, different, stereoscopic views are observed creating, for example, a look-around impression.
FIG. 1 is a schematic perspective view of a known direct view autostereoscopic display device 1. The known device 1 comprises a liquid crystal display panel 3 of the active matrix type that acts as a spatial light modulator to produce the display.
The display panel 3 has an orthogonal array of rows and columns of display sub-pixels 5. For the sake of clarity, only a small number of display sub-pixels 5 are shown in the Figure. In practice, the display panel 3 might comprise about one thousand rows and several thousand columns of display sub-pixels 5. In a black and white display panel a sub-pixel in fact constitutes a full pixel. In a colour display a sub-pixel is one colour component of a full colour pixel. The full colour pixel, according to general terminology comprises all sub-pixels necessary for creating all colours of a smallest image part displayed. Thus, e.g. a full colour pixel may have red (R) green (G) and blue (B) sub-pixels possibly augmented with a white sub-pixel or with one or more other elementary coloured sub-pixels. The structure of the liquid crystal display panel 3 is entirely conventional. In particular, the panel 3 comprises a pair of spaced transparent glass substrates, between which an aligned twisted nematic or other liquid crystal material is provided. The substrates carry patterns of transparent indium tin oxide (ITO) electrodes on their facing surfaces. Polarizing layers are also provided on the outer surfaces of the substrates.
Each display sub-pixel 5 comprises opposing electrodes on the substrates, with the intervening liquid crystal material there between. The shape and layout of the display sub-pixels 5 are determined by the shape and layout of the electrodes. The display sub-pixels 5 are regularly spaced from one another by gaps.
Each display sub-pixel 5 is associated with a switching element, such as a thin film transistor (TFT) or thin film diode (TFD). The display pixels are operated to produce the display by providing addressing signals to the switching elements, and suitable addressing schemes will be known to those skilled in the art.
The display panel 3 is illuminated by a light source 7 comprising, in this case, a planar backlight extending over the area of the display pixel array. Light from the light source 7 is directed through the display panel 3, with the individual display sub-pixels 5 being driven to modulate the light and produce the display. The backlight 7 has side edges 7a and 7b, a top edge 7c and a bottom edge 7d. It has a front face from which light is output.
The display device 1 also comprises a lenticular sheet 9, arranged over the display side of the display panel 3, which performs a light directing function and thus a view forming function. The lenticular sheet 9 comprises a row of lenticular elements 11 extending parallel to one another, of which only one is shown with exaggerated dimensions for the sake of clarity.
The lenticular elements 11 are in the form of convex (semi-) cylindrical lenses each having an elongate axis 12 extending perpendicular to the cylindrical curvature of the element, and each element acts as a light output directing means to provide different images, or views, from the display panel 3 to the eyes of a user positioned in front of the display device 1.
The display device has a controller 13 which controls the backlight and the display panel.
The autostereoscopic display device 1 shown in FIG. 1 is capable of providing several different perspective views in different directions, i.e. it is able to direct the pixel output to different spatial positions within the field of view of the display device. In particular, each lenticular element 11 overlies a small group of display sub-pixels 5 in each row, where, in the current example, a row extends perpendicular to the elongate axis of the lenticular element 11. The lenticular element 11 projects the output of each display sub-pixel 5 of a group in a different direction, so as to form the several different views. As the user's head moves from left to right, his/her eyes will receive different ones of the several views, in turn.
FIG. 2 shows the principle of operation of a lenticular type imaging arrangement as described above in more detail and shows the backlight 20, the display device 24, the liquid crystal display panel and the lenticular array 28 in cross section. FIG. 2 shows how the lenticular 27 of the lenticular arrangement 28 directs the outputs of the pixels 26′, 26″ and 26′″ of a group of pixels to the respective three different spatial locations 22′, 22″ and 22′″ in front of the display device. The different locations 22′, 22″ and 22′″ are part of three different views.
In a similar manner, the same output of display pixels 26′, 26″ and 26′ is directed into the respective three other different spatial locations 25′, 25″ and 25′″ by the lenticular 27′ of the arrangement 28. While the three spatial positions 22′ to 22′″ define a first viewing zone or cone 29′, the three spatial positions 25′ to 25′ define a second viewing cone 29″. It will be appreciated that more of such cones exist (not shown) depending on the number of lenticular lenses of the array that can direct the output of a group of pixels such as formed by the pixels 26′ to 26′. The cones fill the entire field of view of the display device.
The above view directing principle leads to view repetition occurring upon going from one viewing cone to another as within every cone the same pixel output is displayed in a particular view. Thus, in the example of FIG. 2, spatial positions 22″ and 25″ provide the same view, but in different viewing cones 29′ and 29″ respectively. In other words, a particular view shows the same content in all viewing cones. At the boundaries between viewing cones, there is a jump between extreme views, so that the autostereoscopic effect is disrupted.
A solution to this problem is to allow only a single viewing cone, for example by designing the backlight to have a directional output. WO 2011/145031 discloses various approaches for defining a display with a single cone output.
The use of a collimated backlight for controlling the direction from which a view can be seen is for example known for several different applications, including for gaze tracking applications, privacy panels and enhanced brightness panels. One known component of such a collimated backlight is a light generating component which extracts all of its light in the form of an array of thin light emitting stripes spaced at around the pitch of a lenticular lens that is also part of the backlight.
This configuration is shown in FIG. 3 in which the backlight 7 comprises an array 30 of striped light emitters, a positive lens array 32 and a replica structure 34 between the lens array and the emitters. The lens array 32 collimates the light coming from the array 30 of thin light emitting stripes. Such a backlight can be formed from a series of emissive elements, such as lines of LEDs or OLED stripes.
However, such solutions are expensive to fabricate and prone to failure due to a short lifetime in the case of OLEDs.
The broad angular emission pattern of the light emitting elements also illuminates multiple lenses simultaneously, and thereby they still create cone repetition.
Edge lit waveguides for backlighting and front-lighting of displays are inexpensive and robust. It would therefore be advantageous to base a collimated backlight component around the edge lit technology. However the known edge lit waveguides are designed to provide maximum uniformity of light output across the entire surface of the waveguide and are therefore not designed for generating an array of thin light stripes spaced at around the pitch of a combined lens.
FIG. 4 shows a schematic image of an edge lit waveguide 40. The waveguide comprises a waveguide material, such as a slab of solid material with a top face 40a, a bottom face 40b and lateral edges 40c. There are top and bottom edges which cannot be seen in FIG. 4, as the cross section of FIG. 4 is taken in the lateral side-to-side direction. The waveguide is generally rectangular in plan view. The top and bottom edges of the waveguide (at the top and bottom sides of the rectangle) correspond to the top and bottom of the associated display, and the lateral edges (at the left and right sides of the rectangle) correspond to the left and right sides of the associated display. From the left side in FIG. 4, light is coupled in from a light source 42 and at the bottom of the waveguide several out-coupling structures 44 are placed. Light propagates under an angle θin inside the waveguide with height H. The out-coupling structures 44 in this example are drawn as half prisms with a half top angle α, height h, and a width w.
The waveguide is formed as a dielectric slab made out of e.g. glass or polycarbonate. In the slab, total internal reflection at the borders keeps the light confined while the light propagates. The edges of the slab are typically used to couple in light and the small structures 44 locally couple light out of the waveguide.
If it is assumed that light is coupled in on one edge of the waveguide, the out-coupling efficiency of the structures should vary across the waveguide to ensure a homogeneous out-coupling. The out-coupling efficiency of a structure can be tuned in different ways. For example, if the structure resembles a prism-like shape (or half of a prism), the out-coupling efficiency can be modified by changing either the height h of the prism, by changing the half top angle α or by changing both.
The invention is based on the optimization of a waveguide type backlight for use in an autostereoscopic display or privacy display, in particular to enable generation of a striped output.